Method for in-gas micro/nanoimprinting of bulk metallic glass

ABSTRACT

A method for in-gas micro/nanoimprinting of bulk metallic glass includes steps of preparing a die, heating the bulk metallic glass and in-gas micro/nanoimprinting of the bulk metallic glass. In the step of preparing a die, the die has a micro/nano structure having multiple depressions and a flow channel connected to the depressions. In the step of heating the bulk metallic glass, the bulk metallic glass is heated to a temperature between a glass transition temperature and a crystallization temperature of the bulk metallic glass. In the step of in-gas micro/nanoimprinting, the bulk metallic glass is forced into the die in presence of gas to imprint a complementing micro/nano structure on the bulk metallic glass. Because the die has a flow channel to allow air or gas to escape from the micro/nano structure of the die, the micro/nanoimprinting can be performed in presence of air or gas.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to bulk metallic glass, and more particularly to a method for in-gas micro/nanoimprinting of bulk metallic glass that can perform micro/nanoimprinting of bulk metallic glass without using a vacuum system.

2. Description of the Related Art

Superplasticity is the phenomenon that materials undergo very large plastic deformation under tension at specific conditions (proper temperature, formation, strain rate et al.). Generally, superplastic materials have more than about 100% elongation under tensile test.

Superplastic materials can undergo large and uniform plastic deformation without rupture so superplastic materials can be used on industrial manufacturing and molding. Many references and investigation reports indicate that a material exhibits superplasticity and can be machined easily when grain size of the material is small (<10 μm) and the material is at high temperatures (>60% melting temperature) and low strain rates (10⁻⁴-10⁻³ sec⁻¹). Topic of superplasticity is widely studied by many researchers, especially Backofen et al. proposing that superplasticity results mainly from high strain rate sensitivity (m) of flow stress. The strain rate sensitivity (m) can be calculated with flow stress and strain rate sensitivity. The relationship between flow stress (σ) and strain rate ({acute over (ε)}) in a superplastic material can be expressed by the following empirical equation: σ=k{acute over (ε)}^(m), where k is a constant. When a material exhibit superplasticity, strain rate sensitivity (m) of the material is more than 0.3.

Bulk metallic glass (BMG) is also called bulk amorphous metal (BAM) or bulk amorphous alloy (BAA) and has characteristics of high strength, hardness, toughness, anti-corrosion, anti-wear and anti-fatigue. Because bulk metallic glass has high glass-forming ability (GFA) and heat stability, scientific researches and practical applications of bulk metallic glass have advanced significantly.

In comparison with amorphous metal that is formed by rapidly cooling from melting state, bulk metallic glass has a very high glass-forming ability. So bulk metallic glass can be fabricated in bulk at a lower cooling rate and has higher heat stability than conventional metals having a crystalline structure. For example, A. L. Greer (“Metallic Glasses”, Science, 267, 1947(1995)), A. Inoue (“Stabilization of Metallic Supercooled Liquid and Bulk Amorous Alloys”, Acta Mater., 48, 279 (2000)) and W. H. Wang (“Bulk Metallic Glasses”, Material Science and Engineering R, 44, 45-89 (2004)) proposed many bulk metallic glass systems having a high glass-forming ability, such as Zr, Pd, Mg, Fe, Co, Ni, Ti or La based binary, ternary, quaternary or multi-component alloys.

Bulk metallic glass exhibits superplasticity when heated to a supercooled liquid region, between the glass transition temperature and the crystallization temperature. Kawamura et al. (“Superplasticity in Pd₄₀Ni₄₀P₂₀ Metallic Glass”, Scripta Materialia, 39(3), 301(1998)) suggested that Pd₄₀Ni₄₀P₂₀ strip exhibits excellent plastic deformation and has 1260% elongation at strain rate 1.7×10⁻¹ sec⁻¹ and at temperature 620K.

With reference to FIG. 8, the inventor's research of J. P. Chu et al., (“Superplasticity in a Bulk Amorphous Pd-40Ni-20P Alloy: a Compression Study”, Intermetallics, 10, 1191-1195(2002)) described that Pd₄₀Ni₄₀P₂₀ bulk metallic glass has a maximum compressive strain of 0.94 at strain rate 8×10⁻⁴ sec⁻¹ and at temperature 628K as shown in FIG. 8( a).

With further reference to FIG. 9, many inventor's related researches, such as J. P. Chu et al. (“Plastic Flow and Tensile Ductility of a Bulk Amorphous Zr₅₅Al₁₀Cu₃₀N₁₅ Alloy at 700 K”, Scripta Materialia, 49, 435-440 (2003)), C. L. Chiang et al. (“Homogenous Plastic Deformation in a Cu-based Bulk Amorphous Alloy”, Intermetallics, 12, 1057-1061 (2004)) and J. P. Chu et al. (“Compressive Deformation of a Bulk Ce-based Metallic Glasses”, Scripta Materialia, 55, 227-230 (2006)), suggested that Zr, Cu, and Ce based bulk metallic glasses also have excellent compressive or tensile superplasticity as shown in FIGS. 8( b) and 8(c) and can perform micro-forming as shown in FIG. 9.

Saotome et al. (“Superplastic Nanoforming of Pd-based Amorphous Alloy”, Scripta Materialia, 44, 1541-1545 (2001)) use Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass in supercooled liquid region to form the nano V-groove of a V-grooved die of Si with nanoforming. The nanoforming is undergone under a compressive stress of 10 MPa at a maximum temperature of 640K for a working time of 1000 s with a nanoforming apparatus having a compact device and a vacuum system.

Saotome et al. (“The Micro-formability of Zr-based Amorphous Alloys the Supercooled State and Their Application to Micro-die”, Journal of Material Processing Technology, 113, 64-69 (2001)) also use Zr₆₅Al₇₅Cu_(27.5) and La₅₅Al₂₅Ni₂₀ bulk metallic glasses as substrates of a micro-die of Si for micro-forming.

Other micro/nanoforming of bulk metallic glass are stated in following researches: N. H. Pryds (“Bulk Amorphous Mg-based alloys”, Materials Science and Engineering A, 375-377, 186-193 (2004)) and J. Schroers (“The Superplastic Forming of Bulk Metallic Glass”, JOM, 57, 35-39 (2005)). They use Mg and Pt based bulk metallic glasses to fabricate some simple devices with superplastic forming.

Current technology for micro/nanoforming of bulk metallic glass is based on superplasticity of bulk metallic glass. However, conventional micro/nanoforming of bulk metallic glass must be undergone in a vacuum chamber in order to eliminate inclusion of air at the corner of a die. Accordingly, the apparatus performing micro/nanoforming of bulk metallic glass must have a vacuum system and is rather expensive. In addition, processes of performing micro/nanoforming of bulk metallic glass are also complicated.

To overcome the shortcomings, the present invention provides a method for in-gas micro/nanoimprinting of bulk metallic glass to mitigate or obviate the aforementioned problems.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to provide a method for in-gas micro/nanoimprinting of bulk metallic glass that can perform micro/nanoimprinting of bulk metallic glass without using a vacuum system.

A method for in-gas micro/nanoimprinting of bulk metallic glass in accordance with the present invention comprises steps of:

1. preparing a die having

-   -   a micro/nano structure having multiple depressions and a flow         channel being connected to the depressions;

2. heating the bulk metallic glass to a temperature between a glass transition temperature and a crystallization temperature of the bulk metallic glass; and

3. in-gas micro/nanoimprinting of the bulk metallic glass by forcing the bulk metallic glass into the die in presence of gas to imprint a complementing micro/nano structure on the bulk metallic glass.

Other objectives, advantages and novel features of the invention will become more apparent from the following detailed descriptions when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is SEM photos and a schematic diagram of the die in a first embodiment of a method in accordance with the present invention;

FIG. 2 is a schematic diagram of an imprinting device;

FIG. 3 is SEM photos of the dies of Si, the bulk metallic glasses and the PMMAs in first and second embodiments of a method in accordance with the present invention;

FIG. 4 is SEM images of the dies of Si, the bulk metallic glasses and the PMMAs in the first and second embodiments;

FIG. 5 is photos of the appearances of the dies of Si, the bulk metallic glasses and the PMMAs in the first and second embodiments;

FIG. 6 is a schematic diagram of the apparatus for grating diffraction analysis;

FIG. 7 is relative diffraction light strength plots of the dies of Si, the bulk metallic glasses and the PMMAs in the first and second embodiments at different diffraction angles;

FIG. 8 is photos of uncompressed and compressed Pd₄₀Ni₄₀P₂₀ Cu₆₀Zr₂₀Hf₁₀Ti₁₀ and Ce₇₀Al₁₀Cu₂₀ bulk metallic glasses; and

FIG. 9 is SEM photos of Ce₇₀Al₁₀Cu₂₀ bulk metallic glasses with different micro/nano structures.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1 and 2, a method for in-gas micro/nanoimprinting of bulk metallic glass (20) in accordance with the present invention comprises steps of preparing a die (10), heating the bulk metallic glasses (20) and in-gas micro/nanoimprinting of the bulk metallic glass (20). The bulk metallic glass (20) comprises a primary element and at least two secondary elements and has superplasticity at a supercooled liquid region between a glass transition temperature and a crystallization temperature of the bulk metallic glass. The primary element has a high glass-forming ability and may be Cu, Pd, Zr, Ce or Au. The at least two secondary elements are selected from the group consisting of Al, Zr, Cu, Pd, Ti, Ni, Ag, Hf, lanthanide series elements, VIB, VIIB and VIIIB transition metals, P and Si and are different from the primary element. In first and second embodiments of the method in accordance with the present invention, the bulk metallic glass is Pd₄₀Ni₄₀P₂₀.

In the step of preparing a die, the die (10) has a micro/nano structure having multiple depressions and a flow channel (12) connected to the depressions for escaping air or gas from the depressions. In the first and second embodiments of the method in accordance with the present invention, the die (10) is a die of Si and is fabricated by a photolithography comprising steps of coating, soft baking, exposure, development, hard baking and etching. The micro/nano structure formed on the die (10) in the first embodiment as shown in FIG. 1( a) is a grating (11) of 600 μm square with a period of 600 nm fabricated with an E-beam writer. The micro/nano structure formed on the die in the second embodiment is a grating of 600 μm square with a period of 1500 nm fabricated with an E-beam writer.

With further reference to FIG. 1( b), the grating (11) has multiple parallel and equally spaced groove-shaped depressions (111). The flow channel (12) is connected transversely to and is deeper than the groove-shaped depressions (111) for escaping air or gas from the groove-shaped depressions (111). The die (10) further has three guide channels (13) and an outlet (14). The guide channels (13) are 10 μm in width and are connected perpendicularly to the flow channel (12) opposite the grating (10). The outlet (14) is connected to the guide channels (13) to release air or gas.

In the step of heating bulk metallic glass (20), the bulk metallic glass (20) is heated to a temperature between a glass transition temperature and a crystallization temperature of the bulk metallic glass (20). The glass transition temperature and the crystallization temperature of the bulk metallic glass can be calculated by heat analysis with a DSC (Differential Scanning Calorimetry). Before heating the bulk metallic glass (20), the bulk metallic glass (20) is cut to a suitable size with a low-speed cutting machine and the bulk metallic glass (20) is polished sequentially with a sand paper, an aluminum oxide cloth and an abrasion wheel machine. Then the bulk metallic glass (20) is placed in an imprinting device (30) that has a high temperature furnace without a vacuum system and the bulk metallic glass (20) is heated. The die (10) is disposed under the bulk metallic glass (20). A pad (31) is disposed upon the bulk metallic glass (20).

In the step of in-gas micro/nanoimprinting of the bulk metallic glass (20), the bulk metallic glass (20) is forced into the die (10) in presence of gas or air to imprint a complementing micro/nano structure of the micro/nanoimprinting of the die (10) on the bulk metallic glass (20). In the first and second embodiments of the method, the micro/nanoimprinting of the bulk metallic glass (20) is performed under a compressive stress of 10 MPa at a temperature of 650K for an imprinting time of 600 s. Because the groove-shaped depressions (111) on the grating (11) on the die (10) are connected to the flow channel (12), air or gas in the groove-shaped depressions (111) can flow out through the flow channel (12), the guide channel (13) and the output (14) when the micro/nanoimprinting of the bulk metallic glass (20) is performed.

After the bulk metallic glass (20) is cooled down, the bulk metallic glass (20) can serve as a die to reproduce the micro/nano structure on the original die (10) of Si. In the first and second embodiments, a PMMA (40) (polymethylmethacrylate) is placed in the imprinting device (30) and the bulk metallic glass (20) is disposed under the PMMA (40) as a die. Then the PMMA (40) is heated to a temperature of 453K and is forced into the bulk metallic glass (20) to micro/nanoimprint the micro/nano structure on the PMMA (40) under a compressive stress of 10 MPa for an imprinting time of 600 s.

With reference to FIG. 3, the original dies (10) of Si, the bulk metallic glasses (20) and the PMMAs (40) in the first and second embodiments are examined by a SEM (Scanning Electron Microscope). The SEM photos as shown in FIGS. 3( a) and 3(b) show that bulk metallic glass (20) has an excellent formability and the complementing grating of the grating (11) on the die (10) of Si in the first embodiment is imprinting on the bulk metallic glass. The SEM photo as shown in FIG. 3( c) shows a grating on the PMMA (40) similar to the grating (11) on the die (10) of Si and demonstrates that the bulk metallic glass (20) can serve as a die. The SEM photo as shown in FIGS. 3( d), 3(e) and 3(f) show results of the second embodiment same as the first embodiment.

With reference to FIG. 4, the original dies (10) of Si, the bulk metallic glasses (20) and the PMMAs (40) in the first and second embodiments are examined by an AFM (Atomic Force Microscope). The AFM images as shown in FIGS. 4( a), 4(b), 4(c), 4(d), 4(e) and 4(f) show that the bulk metallic glasses (20) have better correspondence relative to the original dies (10) of Si than that of the PMMAs (20) relative to the bulk metallic glasses (20). Grating pitches and depression depths for various gratings in the first and second embodiments are also measured by AFM.

TABLE 1 Grating pitches and depression depths for various gratings measured by AFM. Pitch Depression Shrink- Filling Grating Periodicity age Depth Fraction Type Material (nm) (%) (nm) (%) 600 Die of Si  579.6 ± 37.5 — 201.9 ± 3.5 — BMG  576.8 ± 38.3 0.48 142.0 ± 4.5 70.3 PMMA  572.3 ± 17.8 0.78  129.9 ± 11.2 91.5 (1.26) (64.3) 1500 Die of Si 1487.9 ± 38.2 — 206.1 ± 2.3 — BMG 1484.9 ± 38.1 0.20 202.3 ± 3.9 98.2 PMMA 1469.9 ± 24.5 1.01 190.2 ± 3.1 94.0 (1.21) (92.3)

Note: The values in parentheses are obtained when compared with those of dies of Si.

A summary of AFM surface profile measurements such as grating pitches and depression depths is listed in Table 1. The pitch shrinkages are 0.48% and 0.78%, respectively, for the 600-nm bulk metallic glass and PMMA gratings. The smaller shrinkage of bulk metallic glass than PMMA confirms a better replication property of bulk metallic glass, even though the nanoimprint of bulk metallic glass is performed at a much higher temperature. The superb replication characteristic of bulk metallic glass is further demonstrated in the 1500-nm grating. In this case, the difference in the pitch shrinkage is more significant: a negligible shrinkage of 0.20% in bulk metallic glass as compared to 1.01% in PMMA. The low pitch shrinkage in both 600-nm and 1500-nm gratings of bulk metallic glass is attributable to the structure homogeneity and relatively small thermal expansion in bulk metallic glass. The depression depth of the gratings also provides additional information about the replication property of bulk metallic glass and PMMA. The depression depth in 1500-nm grating on bulk metallic glass is comparable to that of the die of Si with a small difference of 1.8%. Thus, the 1500-nm grating on bulk metallic glass can achieve a 98% groove filling in air for a feature scale of ˜655 nm. This filling rate is exceptional relative to those reported in vacuum. By contrast, the depression depth of the 600-nm grating on bulk metallic glass is about 30% less than that of the die of Si, apparently caused by the larger restraining capillarity force in the finer gratings. In comparison, depression fillings for PMMA in both 600-nm and 1500-nm gratings are in a range of 91%-94%.

With reference to FIG. 5, the appearances of the bulk metallic glasses (20) and the PMMAs (40) of the first and second embodiments give vivid colors of blue, green and red similar to the original dies (10) of Si. Accordingly, the micro/nano structures on the bulk metallic glasses (20) and the PMMAs (40) have a grating capacity.

With reference to FIG. 6, an apparatus (50) for grating diffraction analysis comprises a Hi—Ne laser (51) as a light source. The red light emitted by the Hi—Ne laser (51) is guided by several mirrors (52), passes through a filter (53), an attenuator (54) and a 10 mm focusing lens (55) and hits a grating object (56) to reflect a diffraction light in different diffraction orders. The grating object (56) is mounted on a rotatable device (57) and the grafting object (56) is rotated to reflect the diffraction light in different diffraction orders to a receiver (58). When the diffraction light hits the receiver (58), the relative diffraction light strengths at different diffraction angles are transmitted to a computer (59).

With reference to FIG. 7, the original dies (10) of Si, the bulk metallic glasses (20) and the PMMAs (40) in the first and second embodiments are examined with the apparatus for grating diffraction analysis. The results as shown in the FIGS. 7( a) and 7(b) show that the original dies of Si (10), the bulk metallic glasses (20) and the PMMAs (40) all reflect diffraction lights of very similar diffraction orders at similar diffraction angles. Consequently, the results demonstrate that size and structure of the gratings (11) on the original dies (10) of Si, the bulk metallic glasses (20) and the PMMAs (40) are alike. The gratings of the original die of Si, the bulk metallic glass and the PMMA in the second embodiment reflects diffraction lights of more diffraction orders than that in the first embodiment. According to Manzardo et al. (“Miniature lamellar grating interferometer based on silicon technology,” Optics Letters, Vol. 29, 1437-1439 (2004)), a large grating pitch shrinkage or shallow depression depth in the PMMA gratings can cause peak to shift and the shift becomes more evident for the higher-order diffraction peaks. Similar trend can be observed from the results as shown in FIG. 7. Nonetheless, the differences in peak positions between gratings on the die of Si and the bulk metal glass are insignificant for both embodiments.

Because of superplasticity of the bulk metallic glass (20), micro/nano structures like the grating (11) can be imprinted on the bulk metallic glass (20) with the simple die (10) of Si. The in-gas micro/nanoimprinting is achieved by means of the flow channel (12) on the die (10) of Si so air or gas contained in the grating (11) on the die (10) of Si can flow out through the flow channel (12). The complemented grating on the bulk metallic glass (20) has optic properties similar to those of the grating (11) on the original die (10) of Si. The bulk metallic glass (20) can also be used as a die for a second generation of micro/nano replications. Consequently, the bulk metallic glass (20) has two excellent characteristics: high plastic formability and good die property.

The present invention has the advantages described as following:

1. Because the die (10) for micro/nanoimprinting of bulk metallic glass (20) has a flow channel (12) to allow air or gas to escape from the micro/nano structure on the die (10), the micro/nanoimprinting can be performed in presence of air or gas with an imprinting device (30) without a vacuum system on different bulk metallic glasses (20) like Zr, Pd, Mg, Cu, Au or Ce based bulk metallic glasses (20).

2. Because the micro/nanoimprinting of bulk metallic glass (20) can be performed without a vacuum system, the equipment cost for the micro/nanoimprinting can be reduced and processes of preparing the micro/nanoimprinting can also be simplified.

3. Bulk metallic glass (20) with imprinted micro/nano structure can serve as a die to perform micro/nanoimprinting and replace the brittle die (10) of Si.

Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only. Changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A method for in-gas micro/nanoimprinting of bulk metallic glass comprising steps of preparing a die having a micro/nano structure having multiple depressions and a flow channel being connected to the depressions; heating the bulk metallic glass to a temperature between a glass transition temperature and a crystallization temperature of the bulk metallic glass; and in-gas micro/nanoimprinting of the bulk metallic glass by forcing the bulk metallic glass into the die in presence of gas to imprint a complementing micro/nano structure on the bulk metallic glass.
 2. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 1, wherein the micro/nano structure on the die is a grating of micro/nano-scale period having multiple parallel and equally spaced groove-shaped depressions; and the flow channel is connected transversely to the groove-shaped depressions.
 3. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 2, wherein the flow channel is deeper than the groove-shaped depressions.
 4. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 1, wherein the die further has at lest one guide channel being connected to the flow channel; and an outlet being connected to the at least one guide channel.
 5. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 4, wherein the bulk metallic glass comprises a primary element being selected form the group consisting of Cu, Pd, Zr, Ce or Au; and at least two secondary elements being selected from the group consisting of Al, Zr, Cu, Pd, Ti, Ni, Ag, Hf, lanthanide series elements, VIB, VIIB and VIIIB transition metals, P and Si and being different from the primary element.
 6. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 1, wherein the bulk metallic glass comprises a primary element being selected form the group consisting of Cu, Pd, Zr, Ce or Au; and at least two secondary elements being selected from the group consisting of Al, Zr, Cu, Pd, Ti, Ni, Ag, Hf, lanthanide series elements, VIB, VIIB and VIIIB transition metals, P and Si and being different from the primary element.
 7. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 2, wherein the bulk metallic glass comprises a primary element being selected form the group consisting of Cu, Pd, Zr, Ce or Au; and at least two secondary elements being selected from the group consisting of Al, Zr, Cu, Pd, Ti, Ni, Ag, Hf, lanthanide series elements, VIB, VIIB and VIIIB transition metals, P and Si and being different from the primary element.
 8. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 3, wherein the bulk metallic glass comprises a primary element being selected form the group consisting of Cu, Pd, Zr, Ce or Au; and at least two secondary elements being selected from the group consisting of Al, Zr, Cu, Pd, Ti, Ni, Ag, Hf, lanthanide series elements, VIB, VIIB and VIIIB transition metals, P and Si and being different from the primary element.
 9. The method for in-gas micro/nanoimprinting of bulk metallic glass as claimed in claim 1, wherein the bulk metallic glass is served as a die after the step of in-gas micro/nanoimprinting of the bulk metallic glass. 